As for pre-coated metallic material (i.e., metallic material which is provided with a pre-coating. This is the same in the following description.), many characteristics of coating film such as adhesion on working, corrosion resistance, chemical resistance, coin-scratch resistance, design, etc. are demanded. Therefore, two coat type wherein the undercoating that is called a primer is applied onto a metallic sheet which underwent surface treatment in advance, and a colored topcoating is further applied thereon has been widely used. In such a pre-coated metallic material, the adhesiveness to the metallic sheet is secured by providing the surface treatment, and further the adhesion on working, corrosion resistance, coin-scratch resistance, etc., are given by providing the primer which includes resin and rust preventive pigment between the metallic sheet and topcoating.
As the surface treatment for the pre-coated metallic material, chromate treatment with a treating solution which includes chromic acid, a dichromic acid or those salts as a main ingredient has been used in order to provide a good adhesiveness to the surface of the metallic material and to provide an excellent corrosion resistance to the surface of the metallic material. Further, as for the primer, paints which include chromic rust preventive pigment have been widely used in order to obtain an excellent corrosion resistance. However, in recent consideration of the environmental friendliness, chrome-free treatment or paint wherein another cross-linkable metal is used as a substitute for chrome is practically used today.
In the Patent Literature 1, the disclosed is a process for manufacturing one coat type pre-coated steel sheet which has excellent workability, corrosion resistance, adhesion, impact resistance, and scratch resistance, wherein the process is comprised of coating a paint composition which is prepared by blending a polyester resin, a melamine resin curing agent, a rust preventive pigment, organic polymer minute particles, etc., onto a zinc plated steel sheet which underwent chemical conversion treatment in advance. However, owing to the presence of the rust preventive pigment in the paint composition, the appearance of the coating obtained is compelled to deteriorate. As the result, a problem that various designs can not be given to the pre-coated steel sheet arises. Moreover, since this paint composition is applied to the chromate-treated steel sheet, the process comes off the request in the age.
In the Patent Literature 2, the disclosed is a process for manufacturing one coat type pre-coated steel sheet which has an excellent molding workability, wherein the process is comprised of coating a solvent type paint composition which is prepared by blending a polyester resin, an epoxy resin, a blocked polyisocyanate compound, a melamine resin curing agent, etc., onto a zinc type plated steel sheet which underwent chemical conversion treatment in advance. However, since this paint composition is also applied to the chromate-treated steel sheet, the process comes off the request in the age. In addition, when this paint composition is coated to the steel sheet which underwent chemical conversion treatment including the chromate treatment in advance, the obtained pre-coated steel sheet can not have a sufficient corrosion resistance because the formed organic coating film includes no rust-inhibitor.
In the Patent Literature 3, the disclosed is an aqueous primer-cum-surface treating agent for pre-coated steel sheet which includes an aqueous organic resin ingredient, a condensed sodium phosphate, an aluminum tripolyphosphate, a metallic oxysalt, and a phosphoric acid or an aqueous phosphate, and there is a description that the coating film which excels in the corrosion resistance and the workability can be obtained without using a chromate type rust preventive pigment. However, since the coating film formed with this treating agent does not show sufficient compactness and toughness, the coin-scratch resistance and the chemical resistance of the coating film become insufficient.
In the Patent Literature 4, the disclosed is a coating method which is characterized by coating an aqueous paint composition which includes an acrylic epoxy resin and a rust preventive pigment to a metallic substrate which did not undergo chemical conversion treatment in advance, and by which the coating film having an excellent adhesiveness equal to that of the coating film which is accompanied by a chemical conversion coating film can be formed. However, the adhesion on working of the coating film obtained by this paint composition does not attain the level demanded from the pre-coated steel sheet. In addition, the chemical resistance of the coating film obtained by this paint composition is also insufficient.
In the Patent Literature 5, the disclosed is a metallic surface treating agent which includes a urethane resin, a specific organic compound and zirconium compound, and a metallic material which is prepared by using the above-mentioned agent and which has excellent corrosion resistance, molding workability, chemical resistance and moisture resistance. However, this surface treating agent is intended to be used for the purpose of rust prevention of a metallic material used in a non-painted condition. Therefore, even when this metallic surface treating agent is used for another purpose of forming an undercoating for paint layer, the adhesion on working and the coin-scratch resistance of the coating film obtained by using this surface treating agent does not attain the level demanded from the pre-coated steel sheet.
On the other hand, the methods of obtaining corrosion resistance and lubricity, etc., with various resin ingredients and various additives are disclosed in the Patent Literatures 6-13. Moreover, the adhesiveness with the paint is stated in these inventions. However, the adhesiveness of these invention are the ones at the level of obtaining the adhesion in the cross-cut adhesion test parts, but not the one that the extremely excellent adhesiveness which is provided by the present invention.
Incidentally, since the pre-coated steel sheet necessitates the adhesion on working of the coating film which can endure a severe post-processing such as bending or drawing, a surface treatment coating is usually provided on the steel sheet. The surface treatment coating plays the role to give an excellent adhesion on working of the coating film by linking the upper layers (primer and topcoating) to the surface of metal via the surface treatment coating, whereas a sufficient adhesion of the upper layers directly with the surface of metal can be hardly obtained. Herein, the bending test which is an extremely severe test has been adopted in order to evaluate the adhesion on working of the pre-coated steel.
Further, as for pre-coated steel sheet, corrosion resistance which can endure a long term exposure is required. The corrosion on the pre-coated steel sheet comprises mainly the anodic dissolution at the scratched portions or end face parts, and the generation of blister originated in cathodic reaction which is caused below the coating film at a portion little away from the scratched portion. In the case of the two coat type pre-coated steel sheet, the corrosion resistance given by the surface treatment is implemented by the fact that the coating formed thereby has a strong resistance to acids and a strong resistance to alkalis, and the corrosion resistance given by the primer is implemented by the environmental deprivation ability, such as the low water permeability, of the coating, and the inhibitor effect due to the rust preventive pigment which abundantly contains in the coating. In addition, it is also an important characteristic that an excellent anti-scratching property (coin-scratch resistance) brought by the hard coating of the primer contributes to degression in opportunities of generating starting points of corrosion. The corrosion resistance given by the topcoating is implemented by the environmental deprivation ability due to the thick film thickness of the coating which employs a resin having a strong anti-pollution property.
In addition, various designs are demanded from the pre-coated steel sheet. For instance, the topcoatings according to the purposes, such as a lustrous beautiful appearance, or a mat finish with a low-gloss may be applied.